The present invention relates generally to aerodynamic prediction methods. More specifically, the present invention relates to improved aerodynamic prediction techniques and corresponding methods employing semiemperical code.
The instant application is based on Provisional Patent Application No. 60/259,689, which was filed on Jan. 5, 2001. That Provisional Patent Application is incorporated by reference in its entirety.
The 1998 version of the NSWCDD Aeroprediction Code (AP98) described in the report by Moore, F. G., McInville, R. M., and Hymer, T. entitled xe2x80x9cThe 1998 Version of the NSWC Aeroprediction Code: Part Ixe2x80x94Summary of New Theoretical Methodologyxe2x80x9d (NSWCDD/TR-98/1, April 1998) (Ref. 1) is the most complete and comprehensive semiempirical code produced to date. It includes the capability to predict planar aerodynamics in the roll positions of "PHgr"=0 deg (fins in xe2x80x9c+xe2x80x9d or plus orientation as viewed from the rear of the missile) and "PHgr"=45 deg (fins in xe2x80x9cxc3x97xe2x80x9d or cross roll orientation as viewed from the rear of the missile) over a broad range of flight conditions and configuration geometries with good average accuracy, computational times and ease of use. Flight conditions include angles of attack (AOA) up to 90 deg, control deflections of up to xc2x130 deg, and Mach numbers up to 20. Configuration geometries include axisymmetric and nonaxisymmetric body shapes with sharp, blunt, or truncated nose tips, with or without a boattail or flare. Up to two sets of planar or cruciform fins are allowed.
New technology has recently been developed to allow both six- and eight-fin options in the fin considerations as well. See Ref. 2, i.e., a report by Moore, F. G., McInville, R. M., and Robinson, D. I. entitled xe2x80x9cA Simplified Method for Predicting Aerodynamics of Multi-Fin Weapons,xe2x80x9d (NSWCDD/TR-99/19, March 1999). Moreover, many of the constants used in the aeroprediction code have been refined, as reported by Moore, F. G. and McInville, R. M. in Ref. 3, which is a document entitled xe2x80x9cRefinements in the Aeroprediction Code Based on Recent Wind Tunnel Dataxe2x80x9d (NSWCDD/TR-99/116, December 1999), based on a more recent wind tunnel data base (See also Ref. 4, e.g., Allen, J. M., Hemsch, M. J., Burns, K. A., and Oeters, K. J., xe2x80x9cParametric Fin-Body and Fin-Alone Database on a Series of 12 Missile Fins,xe2x80x9d NASA LRC TM in publication, May 1996.), allowing more accurate aerodynamic estimates at all AOAs. Average accuracies are xc2x110 percent for normal and axial force and xc2x14 percent of body length for center of pressure. Average accuracy signifies that enough AOAs or Mach numbers are considered to get a good statistical sample. It will be noted that, on occasion, a single data point can exceed these average accuracy values.
Ease of use has been significantly enhanced over older versions of the Aeroprediction Code (APC) through a personal-computer-based pre- and post-processor package. See Ref. 5, which is a report by Hymer, T. C., Downs, C., and Moore, F. G. entitled xe2x80x9cUsers Guide for an Interactive Personal Computer Interface for the 1998 Aeroprediction Code (AP98)xe2x80x9d (NSWCDD/TR-98/7, June 1998). This package has allowed inputs for configuration geometries to be simplified significantly by many automated nose shape options.
While the AP98 is a very powerful tool, several limitations and areas of improvement still remain. Most of these needs are driven by the desire of future weapon designers to perform tradeoff studies on new and innovative concepts that may fall outside of the current capability of the AP98. An example of this type of requirement is the multi-fin requirement that has just been completed.
What is needed is an improved APC that includes the capability to model the deflection of the rear segment of a fin (sometimes referred to as flaperon or aileron) for control, as opposed to the entire fin. It would be desirable if the improved APC included the capability of predicting the drag accurately for all power on conditions. It would also be advantageous if the improved APC included the capability to predict the aerodynamics of projectiles that use a flare for stability (as opposed to fins).
Based on the above and foregoing, it can be appreciated that there presently exists a need in the art for improved semiemperical methods for predicting aerodynamic performance which overcome the above-described deficiencies. The present invention was motivated by a desire to overcome the drawbacks and shortcomings of the presently available technology, and thereby fulfill this need in the art.
According to one aspect, the present invention provides an improved aeroprediction code (APC) that allows aerodynamics to be predicted for Mach numbers up to 20 for configurations with flares. Moreover, the improved APC advantageously extends the static aerodynamic predictions for Mach numbers less than 1.2, improves the body alone pitch damping for Mach numbers above 2.0, and develops a new capability for pitch damping of flared configurations at Mach numbers up to 20.
It will be noted that this additional capability for treating flared configurations has been validated for several different configurations in the Mach number range of 2 to 8.8. In general, pitch damping predictions of the improved capability was within 20 percent of either experimental data or computational fluid dynamics calculations. It will be appreciated that this accuracy level is quite adequate for dynamic derivatives generated during the preliminary design stage. It will also be appreciated that these new additions to the aeroprediction code will be transitioned to users as part of the 2002 version of the code (AP02).
According to another aspect, the present invention provides improved methods for base pressure prediction under base bleed and rocket motor-on conditions. More specifically, the base bleed method makes several refinements to the method developed by Danberg at the Army Research Laboratory in Aberdeen, Md. The improved rocket motor-on, base pressure prediction improves upon the method developed at the Army Missile Command in Huntsville, Ala. by Brazzel and some of his colleagues. The major refinement to the base bleed method of Danburg was to estimate the power-off value of base pressure empirically based on an extensive data base, as opposed to using computational fluid dynamics codes to predict this term. The major modifications to the power-on base pressure prediction method of Brazzel was to extend its range of applicability to high values of thrust coefficient, to Mach numbers less than 1.5, and to different afterbody shapes. In comparing the improved methods for power-on base drag prediction to experiment, it was seen that both methods gave reasonable agreement to most experimental data bases. However, more validation is needed, particularly for the combined effects of angle of attack, fins, and power-on conditions.
According to a further aspect, the present invention provides an Improved Semiempirical Method for estimating the static aerodynamics of configurations that use a trailing edge flap for control. The method is based on deflecting the full aft-located lifting surface an amount that allows the normal force coefficient to be equal to that generated by the deflected flap. A transfer in pitching moments and a modified axial force coefficient is derived to complete the set of static aerodynamics. The method is derived using theoretical methods that are a part of the 1998 version of the Naval Surface Warfare Center aeroprediction code and two sets of experimental data. Comparison of the improved method to available data shows the method to give satisfactory results over the practical range that trailing edge flaps are contemplated for use. Additional wind tunnel data advantageously can be employed to further refine and expand the applicability of the disclosed method. This is particularly true for transonic Mach numbers and for supersonic Mach numbers where the angle of attack and control deflection are of the same sign.